Nanostructures to reduce optical losses

ABSTRACT

Methods and systems for creating nanostructures to reduce optical losses are provided. An example described herein provides a solar cell. The solar cell includes an antireflective coating including sloped nanostructures formed in a vapor deposition process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(a) on PatentApplication No. 20210100005, filed in Greece on Jan. 4, 2021, the entiredisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is directed to reducing reflective light losseson the top surface of solar cells.

BACKGROUND

Optical losses in solar cells have a significant impact on theelectrical performance of solar cells. Top surface reflective opticallosses have a remarkable impact on the absorption spectrum of the solarcell and as a result on the photo generated current output. Reducingreflective losses in planar solar cells can be achieved through chemicaltop surface etching. However, chemical etching is challenging for somematerials used in solar cell, such as solution processed materials.Physical texturing of the top surface has been proposed as a solution.However, physical texturing may be uneconomical and not scalable.

SUMMARY

An embodiment disclosed in examples herein provides a method forfabricating sloped nanostructures on a surface of a substrate. Themethod includes placing a mask over the substrate, placing substratewith the mask in a deposition chamber, vaporizing material fordeposition, and allowing clogging of openings in the mask during thedeposition to form sloped shapes.

Another embodiment described in examples herein provides a solar cell.The solar cell includes an antireflective coating including slopednanostructures formed in a vapor deposition process.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic drawings of light reflecting from solarcells that are uncoated and coated with the sloped nanostructuresdescribed herein.

FIG. 2 is a schematic drawing of light transmitted through a structuralcoating of sloped shapes to the surface of an underlying solar cell.

FIG. 3 is a schematic drawing of light transmitted through a structuralcoating of sloped shapes wherein each of the shapes overlaps to form abase layer over the solar cell.

FIGS. 4A and 4B are schematic diagrams of projected evaporation onto asubstrate to form a sloped nanostructures.

FIG. 5 is a drawing of a mask that can be used to form pyramidal shapeson the substrate.

FIG. 6 is a drawing of a mask that can be used to form conical shapes ona substrate.

FIG. 7 is a process flow diagram of a method that can be used to formsloped nanostructures on a surface of a substrate.

DETAILED DESCRIPTION

As described herein, some types of solar cells are difficult to formwith textured surfaces, such as solution processed solar cells.Embodiments described in examples herein provide a technique offabricating three dimensional structures, termed sloped nanostructures,over a top surface of a solar cell to increase internal reflections and,thus, total absorption by the solar cell. A highly transparent andsloped three dimensional structure is patterned on the top surface ofplanar solar cells through projected physical evaporation.

FIGS. 1A and 1B are schematic drawings of light reflecting from solarcells that are uncoated and coated with the sloped nanostructuresdescribed herein. FIG. 1A shows a solar cell 102, for example, formed bya solution process technique, which does not have the antireflectionstructures described herein. An incident light ray 104 hits the surfaceof the solar cell 102, and a portion of the photons are reflected awayin a reflected light ray 106. In the schematic drawings of FIGS. 1A and1B, the relative length of the reflected light ray 106 to the incidentlight ray 104 are used to provide a general indication of the number ofphotons that are reflected from the surface of the solar cells 102 and108.

FIG. 1B shows a solar cell 108 that has the antireflection structuresdescribed herein formed on the surface. In this example, an incidentlight ray 110 is reflected from the surface of the solar cell 108,forming a reflected light ray 112 that includes fewer photons than theincident light ray 106 impacting the smooth surface.

FIG. 2 is a schematic drawing of light transmitted through a structuralcoating 202 of sloped shapes to the surface 204 of an underlying solarcell 206. Light projected on the solar cell is mostly transportedthrough the structural coating 202 to the active layers of the solarcell 206. The structure coating. 202 is selected to be highlytransparent material that has the same or higher transmission spectrumof the material forming, the surface 204 of the solar cell 206. Further,the transparent material may be selected to have a refractive indexsimilar to the surface 204 of the solar cell 206, and a transmittancespectrum similar or higher than the materials used to encapsulate thesolar cell, which is often glass. In some embodiments, the evaporatedmaterial used for the structural coating 202 is silicon nitride. Siliconnitride is a highly transparent material that matches the transmittancespectrum of glass, which is often used as a top surface in theencapsulation of solution processed solar cells, e.g., over thestructure coating 202.

The structural coating 202 increases the total internal reflectance overthe surface 204, reducing interlayer refractive reflection losses. Forexample, a light ray 208 impinging on 1 of the structures is transmitted210 towards the surface 204 of the solar cell 206. In some cases, partof the light ray may be reflected 212 back from the surface 204, forexample, due to the incident angle or the refractive index mismatch ofthe different layers of the solar cell 206. Therefore, it is highlyadvisable to match refractive index of the structural coating 202 to thesolar cell 206. The angled surface of the structure allows the reflected212 light to impinge on the surface of the structure at a low anglecausing an internal reflection 214. The light ray reflected off thesurface by internal reflection 214 impinges on another surface of thestructure and is directed towards the surface by a second internalreflection 216, and is then absorbed by the solar cell 206. As anotherexample, a second light ray 218 impinges on the surface of a structureat an oblique angle. A portion 220 of the light ray 218 is directedtowards the surface 204 of the solar cell 206 where it is absorbed.Another portion 222 of the light ray 218 is reflected off the surface ofthe structure, impinging on a second structure, At the second structure,the portion 222 of the light ray 218 is transmitted 224 towards thesurface 204 of the solar cell 206, where it is absorbed. Accordingly,the deposition of the nanostructures reduces the light reflecting fromthe surface 204 of the solar cell 206. As described with respect toFIGS. 5 and 6 the structures may be conical pyramids or square pyramids,depending on the mask chosen to form the structures.

As the antireflection properties of the structural coating 202 is basedon total internal reflectance, the properties may be adjusted bychanging the angle 226 of the structures. This may be used, for example,to adjust the efficiency of the structural coating 202 to match theexpected range of angles of light impinging on the solar cell 206.

FIG. 3 is a schematic drawing of light transmitted through a structuralcoating of sloped shapes wherein each of the shapes overlaps to form abase layer 302 over the solar cell. Like numbered items are as describedwith respect to FIG. 2 . As described herein, the process to form theshapes may be adjusted, for example, depending on such factors as theexpected range of angles for the radiation to the solar cell, thematerials of the solar cell, and the like. In the example of FIG. 3 ,each of the shapes overlap, creating a base layer 302 that is offsetfrom the surface 204 of the solar cell 206. The height 304 of the baselayer 302 may be adjusted by the amount of overlap. Increasing theheight 304 of the base layer 302 may be used to adjust the amount oftotal internal reflectance, for example, based on the expected range ofangles for the radiation impinging on the solar cell.

FIGS. 4A and 4B are schematic diagrams of projected evaporation onto asubstrate 402 to form a sloped nanostructures 404. As described hereinthe substrate 402 in some embodiments the substrate is a solar cell. Inthe projected evaporation process, material is evaporated from anevaporation source 406. The evaporated material 408 passes through amask 410 that comprises nanoscale openings 412. The evaporated material408 is directed at a deposition spot 414 that is directly below one ofthe nanoscale openings 412, may be about the same size as the nanoscaleopening 412 at the beginning of the process, decreasing as the nozzleclogs. The mask 410 is positioned at a distance 416 from the depositionspot 414, which results in uneven deposition of the evaporated material408, higher at the center and lower at the sides of the deposition spot414.

The evaporated material 408 is also deposited on the mask 410 and at theedges of the nanoscale openings 412 in the mask 410. Together with themigration effect the deposition of material at the edges of thenanoscale openings 412 slowly clogs the nanoscale openings 412 of themask 410. As the evaporative deposition proceeds, the cloggingincreases, which reduces the size of the nanoscale openings 412 in themask 410, and thus reduces the size of the deposition spot 414. Theresulting evaporated structure is a sloped nanostructure, such as aconical shape or a pyramidal shape, among others.

The distance 418 between the evaporation source 406 and the mask 410,and the size of the evaporation source 406 determines the angle 226(FIG. 2 ) of the wall and the size of the base of the slopednanostructure 404. For example, as the distance 418 between theevaporation source 406 and the mask 410 is decreased, the base of thesloped nanostructure 404 and the angle 226 are increased, e a the slopednanostructure 404 has less steep walls. The distance 418 is tuned tocontrol the parameters of the sloped nanostructure 404, for example, sothat the angle 226 of the wall of the sloped nanostructure 404 is notlarge. The distance 420 between the nanoscale openings 412 on the mask410 is generally selected to be equal to or less than the nanoscaleopenings 412, for example, less than about 500 nm to ensure full surfacedeposition. This may be used to prevent gaps between slopednanostructures 404. However, the distance may be increased to change theshape of the sloped nanostructures 404. A larger distance 416 increasesthe width of the area of projected evaporation and the size of the baseof the sloped structure 414. The evaporation rate, the distance 416between the mask 410 and the deposition spat 414, and the size of thenanoscale openings 412 in the mask 410 determines the height of thesloped nanostructure 404. In some embodiments, the distance 416 isbetween about 1 μm and about 2 μm. This distance 416 promotes theformation of a thin structural coating 202 (FIG. 2 ) and is far enoughto protect the tip of the sloped nanostructure 404 from being damaged bythe mask 410, for example, as the mask 410 is removed.

The mask opening 412 could be made smaller or larger depending on thedesired base area 414 of the sloped structure 404 and its height. Invarious embodiments, the openings are between about 100 nm and about 10μm. In some embodiments, the size of the nanoscale openings 412 is about500 nm. A narrow opening results in a short and small sloped structures,and a wide opening results in a large sloped structures. The use of theclogging affect to form the sloped nanostructures is shown in FIG. 4B.To promote the start of the clogging, in some embodiments, the design ofthe mask 410 includes edges 422 of the nanoscale openings 412 that arehigher than the rest of the mask 410. Creating tip-shaped corners at thenanoscale openings 412 promotes migration and the formation of clogging424. The clogging 424 is used to create the sloped nanostructures 404.

FIG. 5 is a drawing of a mask 500 that can be used to form pyramidalshapes on the substrate. The mask 500 has square nanoscale openings 502,which, as the clogging 424 (FIG. 4 ) proceeds, creates slopednanostructures that have a pyramidal shape.

FIG. 6 is a drawing of a mask 600 that can be used to form conicalshapes on a substrate. The mask 600 has round nanoscale openings 602,which, as the clogging 424 (FIG. 4 ) proceeds, creates slopednanostructures that have a conical shape.

FIG. 7 is a process flow diagram of a method 700 that can be used toform sloped nanostructures on a surface of a substrate. As describedherein, in some embodiments, the sloped nanostructures are formed on thetop surface of a solar cell to provide total internal reflections thatdecrease the amount of light reflecting off the top surface withoutbeing absorbed. In these embodiments, the tips face outward from thesurface of the solar cell. In another embodiments, the slopednanostructures are formed behind light sources used in a liquid crystaldisplay panel to increase the amount of light from the light sourcesthat is directed to the panel. In these embodiments, the tips faceoutward from the light sources to increase total internal reflectiontowards the liquid crystal panel.

The method begins at block 702, with a determination of the separationof the mask and substrate. As described herein, the separation may beused to control the shape of the sloped nanostructures, including theangles between the walls, the widths of the base, and the overlappedbetween adjacent sloped nanostructures.

At block 704, a determination is made of the separation of the mask inthe evaporative source. As described herein, the separation between themask and source also provides control over the shape of the slopednanostructures.

At block 706, the mask is placed over the substrate. As describedherein, in some embodiments, the substrate is a solution process solarcell. In some embodiments, the mask is held apart from the substrate byan open frame with a thickness corresponding to the separation betweenthe mask in the substrate.

At block 708, the substrate with the mask is placed in a depositionchamber. The deposition chamber is generally a high vacuum chamber witha heated source providing the vapor for deposition. In some embodimentsdescribed herein, the separation between the source and the mask isadjustable, for example, to control the shapes of the slopednanostructures.

At block 710, the material is vaporized for deposition. This may beperformed by heating a filament behind the material, or using othertechniques to heed the material to vaporize it in the vacuum of thedeposition chamber.

At block 712, clogging of the mask is allowed during the deposition toform the sloped shapes. In contrast to other vapor deposition processes,the deposition may be allowed to proceed for a longer period of time,for example, at a lower rate. The resulting clogging leads to theformation of the sloped nanostructures, as described herein.

At block 714, the mask and substrate are removed from the depositionchamber, for example, after the chamber is allowed to cool and thevacuum is released. In some embodiments, a continuous roll of materialis treated in a deposition chamber.

An embodiment disclosed in examples herein provides a method forfabricating sloped nanostructures on a surface of a substrate. Themethod includes placing a mask over the substrate, placing substratewith the mask in a deposition chamber, vaporizing material fordeposition, and allowing clogging of openings in the mask during thedeposition to formed sloped shapes.

In an aspect, the method includes selecting a separation of the mask andthe substrate to control a slope of the sloped nanostructures. In anaspect, the method includes separating the mask and the substrate byabout 1 (micrometer) μm to about 2 in an aspect, the method includesseparating the mask and the substrate by about 0.5 μm to about 1 μm.

In an aspect, the method includes forming tips at edges of the openingsin the mask to enhance clogging during deposition. In an aspect, themask includes round openings to form conical nanostructures. In anaspect, the mask includes square and triangular openings to formpyramidal nanostructures. In an aspect, the mask includes slits to formridges.

In an aspect, the method includes vaporizing silicon nitride to form thesloped nanostructures.

In an aspect, a solar cell is the substrate. In an aspect, a back panelof a liquid crystal display is the substrate.

Another embodiment described in examples herein provides a solar cell.The solar cell includes an antireflective coating including slopednanostructures formed in a vapor deposition process.

In an aspect, the sloped nanostructures include conical nanostructures.In an aspect, the sloped nanostructures include pyramidalnanostructures. In an aspect, the sloped nanostructures include ridges.

In an aspect, the sloped nanostructures include an angle of less than45° along a cross-section through a tip of the sloped nanostructures. Inan aspect, the sloped nanostructures include an angle of greater than45< along a cross-section through a tip of the sloped nanostructures.

In an aspect, the sloped nanostructures are disposed at about 500 nm ofseparation. In an aspect, the sloped nanostructures are between about200 nm and about 450 nm in height. In an aspect, the slopednanostructures are between about 550 nm and about 750 nm in height. Inan aspect, the sloped nanostructures overlap to form a base layer overthe solar cell. In an aspect, the sloped nanostructures include siliconnitride.

Other implementations are also within the scope of the following claims.

1.-11. (canceled)
 12. A solar cell comprising an antireflective coatingcomprising sloped nanostructures formed in a vapor deposition process.13. The solar cell of claim 12, wherein the sloped nanostructurescomprise conical nanostructures.
 14. The solar cell of claim 12, whereinthe sloped nanostructures comprise pyramidal nanostructures.
 15. Thesolar cell of claim 12, wherein the sloped nanostructures compriseridges.
 16. The solar cell of claim 12, wherein the slopednanostructures comprise an angle of less than 45° along a cross-sectionthrough a tip of the sloped nanostructures.
 17. The solar cell of claim12, wherein the sloped nanostructures comprise an angle of greater than45° along a cross-section through a tip of the sloped nanostructures.18. The solar cell of claim 12, wherein the sloped nanostructures aredisposed at about 500 nm of separation.
 19. The solar cell of claim 12,wherein the sloped nanostructures are between about 200 nm and about 450nm in height.
 20. The solar cell of claim 12, wherein the slopednanostructures are between about 550 nm and about 750 nm in height. 21.The solar cell of claim 12, wherein the sloped nanostructures overlap toform a base layer over the solar cell.
 22. The solar cell of claim 12,wherein the sloped nanostructures comprise silicon nitride.